In automobiles the protection of passengers in case of an impact by means of so-called airbag systems has meanwhile become a standard feature. It is common practice to introduce a recess into the dashboard or the instrument panel, respectively, in front of the co-driver, into which the device for the airbag is placed. For optical reasons it is common that the airbag devices are mounted invisibly. For this purpose, a material weakening is provided for the airbag of the co-driver from the rear side in the region of the instrument panel, which supports the airbag device, which weakening is sufficient to make the visible surface tear open and thereby release the airbag in the case of an accident.
The introduction of material weakenings is already known from the prior art, wherein, in the case of multi-layered instrument panels, the uppermost skin is weakened while the layers of the multipart instrument panel provided underneath the same are completely broken through. However, if the instrument panel is made of one single piece, no satisfactory solution has yet been found with respect to the introduction of a necessary weakening. In the production of injection-molded instrument panels, attempts to provide a weakening already in the injection-molded part, into which extremely thin-walled regions are introduced, have failed. This is, above all, due to the fact that the weakened regions become inevitably visible because the pressure ratios and flow behaviors in the region of extremely thin-walled spots differ to such an extent that flow marks and flow lines make the visible surface unattractive.
Previous attempts to produce a weakening in the injection-molded instrument panels from the rear side in a subsequent working cycle by means of milling a groove or a slot have not brought about any useful results, however, since the injection-molded part does not rest closely enough against the supporting mold or support. The aforementioned visible surface of the instrument panel reliably adjusts to the reference surface of a vacuum-supplied receptacle as a soft, conformable skin under the force of the air pressure. Given a rigid injection-molded part a reliable contact cannot be assumed, however, due to the molded-in stress. Accordingly, the above-mentioned, previously common processing methods fail.
An additional aspect in view of the safety is that weakened lines have to be introduced into instrument panels with a small tolerance so as to preserve a so-called residual wall thickness. The residual wall thickness is tolerated such that, on the one hand, a secure tear-out takes place in case of an accident while, on the other hand, this weakening cannot result in a visible line or even in an unintended tear-out beyond the service life of the vehicle. Accordingly, usual residual wall thickness amount to approximately 0.5 mm, and usual tolerances for these residual wall thicknesses range in the order of +/−0.03 mm. In accordance with this, it is important for the high-precision introduction of weakened lines particularly into an instrument panel of an automobile, that an exact control of the length and form of the processing tool is performed, that the tolerance of the involved axes and constructional units of the machines is observed with respect to the tool, and that the position of the support carrying the work piece or component or molded part, respectively, is taken into account.